(1) Field of the Invention
The present invention relates generally to the interface between an Internet protocol (IP) network and a synchronous optical network/synchronous digital hierarchy (SONET/SDH) network, and more particularly to a transmitter, a SONET/SDH transmitter, and a transmission system suitable for line protection meeting Ethernet of 1 Gbit/sec and 10 Gbit/sec.
(2) Description of the Related Art
Recently, with the demand for the higher-speed operation of a local area network (LAN), the standard of 1 Gbit/sec Ethernet (hereinafter referred to as 1 Gbps Ethernet or 1 GbE) has spread, and a high-speed LAN optical interface called 10 Gbit/sec Ethernet (hereinafter referred to as 10 Gbps Ethernet or 10 GbE) has been examined as the next generation LAN standard.
In the IP network, a voice over Internet protocol (hereinafter referred to as a VoIP) for transmitting voice by employing IP packets (IP datagrams) is also being used. An IP router employing this VoIP is a device which forwards voice information such as a telephone call, and is being utilized with ever-increasing frequency.
In addition, a core network is positively being introduced that is capable of transmitting high-speed and large-volume data by wavelength division multiplexing (WDM), using optical signals. This core network is equivalent to a large-scale wide area network (WAN) when viewed from the LAN. In the core network, techniques as WAN are employed. The fundamental technique that is employed in the core network is SONET/SDH.
The SONET/SDH is one of the architectures in an optical transmission system and refers to a network in which a great number of SONET/SDH transmitters are interconnected through optical fiber cables (hereinafter referred to as optical fibers) so that they are synchronized with one another. Because of this, a technique for connecting the LAN directly with a SONET/SDH network has been started to enhance the throughput of the entire network, and in future, the connection between the LAN and the SONET/SDH network will be increased.
The SONET/SDH network is requested to have line reliability. In the Bellcore (GR-253) and the International Telecommunication Union-Telecommunication (ITU-T), automatic protection switching (APS) and multiplex section protection (MSP) are recommended as line protection systems.
The APS and MSP are principally stipulated for the line protection of paths multiplexed between transmitters provided opposite each other in a network. For line protection, two or more lines, an active line and a standby line, are provided. The active line is also called the working line. The standby line is also called the protection line. In the following description, “WK” means a working line or a working line side and “PT” means a protection line, a protection line side, or a standby system. For the switching structure, a (1+1) structure and a (1:N) structure (where N represents an integer of 2 or greater) are primarily employed.
FIG. 7A shows the (1+1) structure and FIG. 7B the (1:4) structure. The transmitters 400, 500 shown in FIG. 7A are connected through two transmission lines WK and PT. If a fault occurs at the transmission line WK, the transmission line PT operates as a working line. The transmitters 400, 500 shown in FIG. 7B are connected through four transmission lines WKs and a single transmission line PT. If a fault occurs at one of the four transmission line WKs, the single transmission line PT functions as a working line.
FIG. 31 schematically shows a transmission system that has a redundancy structure. The transmission system shown in the figure is equipped with subscriber networks 201, a SONET/SDH network 102, IP routers (VoIP routers) 231, an IP network (e.g., Internet) 204, and LANs 205.
The subscriber network 201 refers to a telephone network, an integrated services digital network (ISDN), an asymmetric digital subscriber line (ADSL) network, and a network having high-speed digital lines or these subscriber terminals.
The SONET/SDH network 102 is a core network to which the SONET/SDH system is applied, and has optical transmitters 300 that are connected with the IP routers 231. The SONET/SDH network 102 is also equipped with a great variety of line protection functions as the fault healing functions (self-healing functions). Examples of systems having the line protection function are a unidirectional path-switched ring (UPSR), a bidirectional line-switched ring (BLSR), etc. By employing these systems, line switching can be completed within 50 ms, even if a fault occurs.
The IP router 231 transmits IP packets to the SONET/SDH network 210 and also to the IP network 204, and has an Ethernet interface cards to interface with the SONET/SDH transmitter 300 provided with 1 Gbps/10 Gbps Ethernet cards. Note that the expression “1 Gbps/10 Gbps” used herein sometimes means both “1 Gbps and 10 Gbps” and “1 Gbps or 10 Gbps.”
This Ethernet standard is stipulated in the Institute of Electrical and Electronics Engineers (IEEE) standard 802.3. In addition, the LAN 205 is a private network provided, for example, in an enterprise, a school, etc. The IP network 202 is a network to which the IP protocol is applied.
In this manner, the transmission system 200 is constructed with the SONET/SDH network 102 as the center. In the part of the transmission system 200 that connects the IP router 231 and the SONET/SDH network 102 together, no packet loss or delay is allowed, because an important IP packet, such as voice data, etc., flows. Therefore, a redundancy structure is required.
Next, a description will be given of the conventional SONET/SDH network 102, IP network 204, and redundancy structure that have been proposed.
(X-1) Configuration employing a Special Reuse Protocol (SRP)
FIG. 32 schematically shows a ring-configured network 210 to which SRP is applied. The ring-configured network 210 shown in the figure includes SRP units 211a, 211b, 211c, and 211d, which are interconnected in ring form. The SRP units 211a, 211b, 211c, and 211d are interconnected through two rings, a counterclockwise ring (inner ring) and a clockwise ring (outer ring).
By employing layer 2, control packets (SRP packets) are transmitted onto the inner ring of the ring-configured network 210, and data packets are transmitted onto the outer ring. Note that the layer 2 used herein means a media access control (MAC) layer.
FIG. 33 shows a format for the SRP packet. The SRP packet shown in the FIG. 33 has a TTL (time to alive) field, a RI (SRP Ring Identifiers) field, a mode field, a priority field, and a parity check field, and also has an MAC (Media Access Control) header attached to the first part thereof. The RI field is a SRP identifier, and the mode field is an identifier for a control packet, a data packet, etc. The priority field represents a packet priority value (0, 1, 2, 3, 4, 5, 6, or 7), and the parity check field represents odd parity.
FIG. 34 shows how transmission in the SRP ring-configured network is performed when a fault occurs. The term “fault” used herein means a transmission line fault due to cutting of an optical fiber, a fault in a device (such as a transmitter, a repeater, etc.) due to an interface card failure, etc., an increase in a bit error rate, etc. If a line disconnection (link cutting, a line disconnection, a communication disconnection, or interruption) occurs due to the occurrence of a fault between the SRP units 211a and 211b shown in FIG. 34, then the SRP units 211a, 211b both send back an optical signal detecting the line disconnection. That is, transmission becomes possible by employing the inner and outer rings.
(X-2) IP Network Redundancy Structure (Virtual Router Redundancy Protocol (VRRP))
The VRRP is a protocol standardized by the Internet Engineering Task Force (IETF) and is applied to a transmission system that has a virtual router section consisting of a plurality of virtual routers. In the VRRP, when a fault occurs in a working router, the fault is detected and the damaged router is quickly switched to a redundant router. An example of a protocol that is applied to the router belonging to the LAN 205 shown in FIG. 31 will be described with reference to FIGS. 35 and 36.
FIG. 35 schematically shows how the virtual router section is operated according to the VRRP when there is no fault. The system shown in the figure is equipped with a WAN 205b, a virtual router section 221 having first and second routers 221a, 221b, and a LAN 205a. The first and second routers 221a, 221b are equipped with interface cards (not shown) having IP addresses A and B, respectively. The virtual router section 221 employs the IP address A of the first router 221a as its IP address. During normal operation, the first router 221a operates as a working router.
FIG. 36 schematically shows how the virtual router section is operated according to the VRRP when a fault occurs. If a fault occurs at the first router 221a shown in FIG. 36, a VRRP packet cannot be transmitted. Because of this, the damaged first router 221a is automatically switched to the second router 221b and therefore communication is continued. When this is occurring, the second router 221b uses the IP address A of the damaged router 221a as its IP address and operates as a working router.
(X-3) Redundancy Structure in the SONET/SDH System
Next, a description will be given of the line protection function. The line protection function is the function of healing a fault, such as a transmission line fault, a device failure, and an increase in a bit error rate, and protecting a line. The SONET/SDH system employs automatic protection switching (APS) and multiplex section protection (MSP) which exhibit the self-healing function by switching. As an example of this self-healing function, a linear (1+1) APS structure (see FIGS. 7A and 7B) and the function thereof will be described with reference to FIG. 37.
FIG. 37 shows the (1+1) APS structure employing the SONET/SDH system. The SONET/SDH units (SONET/SDH transmitters) 230a, 230b shown in the figure are units applied to the SONET/SDH network 102 and are connected opposite each other through optical fibers. The SONET/SDH unit 230a is provided with a pair of interface units 250a, 250b to process a SONET/SDH frame. Similarly, the SONET/SDH unit 230b is provided with a pair of interface units 250c, 250d. The interface units 250a, 250c function as WKs and the interface units 250b, 250d as PTs. The full duplex transmission and reception of optical signals are performed by two systems.
With such a structure, during normal operation, the interface units 250a, 250c are selected as WKs and the interface units 250b, 250d as PTs. A signal that is transmitted by the SONET/SDH unit 230a is split at a distributing section 250e so that the same signals can be transmitted onto parallel optical fibers connected with the interface units 250a, 250b. One of the two same signals received by the interface units 250c, 250d of the SONET/SDH unit 230b is selected at a selecting section 250f and output as a signal on the receiving side.
On the other hand, when a fault occurs at WK, the transmission line is switched quickly from WK to PT to maintain the communication line. Note that a revertive mode can be set so that when a fault at the WK is recovered, the transmission line reverts to the WK again. In addition, a non-revertive mode can be set so that the transmission line does not revert to the WK again.
This switching is performed by employing a K-byte of data. This K-byte of data represents transmission-line switching control information and is employed for the multiple section switching defined in the overhead byte (OHB) field of the SONET/SDH frame. The APS function or MSP function is exhibited by employing the K-byte of data.
Note that the OHB is transmitted and received in transmission cycles (125 μs) for the SONET/SDH frame. When a fault occurs, the completion of switching from a switching command (K-byte) is performed quickly in a short time within 50 ms. The standard of the K-byte of data is defined as 2 bytes of data (K1-byte and K2-byte) contained in the line overhead (LOH) field. The K-byte in the SONET is stipulated in GR-253 and the K-byte in the SDH is stipulated in the G.783 of ITU-T.
FIG. 38A shows a format example of the K1-byte. This K1-byte consists of 8 bits between b1˜b8. The first 4-bit part (b1 through b4) represents a request message type. The last 4-bit part (b5 through b8) represents a channel number from which the request message was transmitted.
FIG. 38B shows a format example of the K2-byte. The first 4-bit part (b1 to b4) of the K2-byte data employs the same code as the K1-byte and represents a channel number at which bridge action was performed. The b5 in the K2-byte identifies the (1+1) or (1:N) redundancy structure, and the last 3-bit part (b6 to b8) represents AIS-L (111), etc.
Note that the line switching procedure employing the K1-byte and K2-byte is stipulated in the SONET/SDH system and makes an interconnection between different benders possible. In addition, the code definition of the K-byte varies between various redundancy structures ((1+1) structure/(1:N) structure, bidirectional/unidirectional, and reversible/irreversible).
A conventional method of interconnecting LANs and the SONET/SDH network 102 is realized by a packet-over-SONET (POS) technique.
(X-4) Redundancy Structure in an IP Transmitter (IP Over SONET Unit) on the SONET
FIG. 39 shows a SONET/SDH transmitter (with the POS function). A router 231 shown in the figure incorporates a SONET interface and has interface units 231a to 231d. The interface units 231a to 231d are disposed between an IP network 204 and a SONET/SDH network 102. For instance, the interface units 231a, 231b can meet the speeds of 1 Gbps Ethernet and 10 Gbps Ethernet. The interface units 231c, 231d are connected with the SONET/SDH network 102 and can meet both standard OC-192c (optical carrier 192) and standard OC-768c (optical carrier 768). By employing the POS in this technique, an IP packet that is transmitted on the side of a LAN is mapped into a SONET/SDH frame. Note that it is recommended that Ethernet of 10 Gbps be standardized by IEEE standard 802.3ae Task Force.
A major difference between Ethernet of 10 Gbps and Ethernet of 100 Mbps and 1 Gbps is in the following (Y-1) to (Y-3).
(Y-1) Ethernet of 10 Gbps supports only a full duplex transmission system. It does not use a CSMA/CD (carrier sense multiple access with collision detection) system.
(Y-2) For transmission media that are employed in 10 Gbps Ethernet interfaces, only optical fibers are used.
(Y-3) For the transmission media of 10 Gbps Ethernet interfaces, WAN-PHY is stipulated. A stipulation for this WAN-PHY is standardized on the assumption that it is interchangeable with SONET OC-192c/SDH VC-4-64.
FIGS. 40A to 40D show format examples of link status signals, respectively. Shown in FIG. 40A are the elements of a link status. Also shown in FIG. 40B is a protocol. These are all applied to 100 Mbps/1 Gbps Ethernet. The elements and protocol shown in FIGS. 40C and 40D are applied to 10 Gbps Ethernet. That is, for the WAN-PHY of 10 Gbps Ethernet, it is being examined that part of the overhead field of the SONET/SDH frame format is inserted into the information byte. With these formats, the transmitting side performs transmission at predetermined time intervals. For example, the transmitting side inserts an inter-packet gap between two incoming IP packets.
For a redundancy structure in the IP network 204 that is constructed in FIG. 39 by LANs, redundancy based on route-switching information is possible by monitoring a fault between a plurality of routers according to VRRP, and employing a routing protocol such as OSPF, BGP, RIP, etc.
Because of this, the interface unit 231a needs to have a routing table. This routing table is used to route IP packets and manage IP addresses.
The time required for the interface unit 231a to process a fault is about a few seconds to a few minutes until the protocol converges and recovers. That is, a management table of IP addresses is necessary so that a data frame transmitted from the SONET/SDH network 102 can be transmitted to the IP network 204 as IP packets.
Furthermore, if optical fibers, etc., are physically cut, it will take a few seconds to a few minutes for the transmission line to be recovered. If a fault occurs at an optical fiber connected to an IP router near a core system, the influence of the line disconnection will be considerable.
Next, for the existing network with IP routers, a description will be given in the case where there is one transmission line between IP routers and the case where there are two transmission lines, with reference to FIGS. 41 to 44.
(Z-1) Case of one transmission line being between routers:
FIG. 41 shows normal operation of routers when there is one transmission line between the routers. An IP network 204 shown in the figure is equipped with IP routers A, B, and C; LANs A, B, and C connected to the IP routers A, B, and C; and transmission lines 241a, 241b, and 241c. 
The LANs A, B, and C are, for example, private networks for enterprises, respectively. Although not shown, each LAN has network terminations (hereinafter referred to as NTs). The IP routers A to C each have a routing table in which IP addresses (destinations) and port (physical port) names are held so that they correspond to each other. Based on the held data, each IP router determines a route and transmits an IP packet 243.
The transmission lines 241a to 241c are capable of transmitting IP packets of 100 Mbit/sec (Mbps) and 1 Gbit/sec (Gbps) by full duplex transmission. Thus, the bandwidth of this IP network 204 is 100 Mbps/1 Gbps×2 (full duplex).
FIGS. 42A to 42C show examples of routing tables when normal operation is performed with one single transmission line. In the routing tables, the left column indicates a destination and the right column an IP router. These routing tables 242a to 242c are generated based on a dynamic routing protocol (an interior protocol (RIP 2, OSPF), an exterior protocol (BGP, EGP), etc.). According to the routing table, an IP packet reaches its destination via determined routes.
In such a structure, an NT within the LAN C shown in FIG. 41 transmits an IP packet 243 addressed to the LAN B. This IP packet 243 has a destination address (DA) and a source address (SA) and is transmitted to the IP router C disposed within the network domain of the LAN C. The IP router B transfers the IP packet 243 to the LAN B, based on the routing table.
FIG. 43 shows how routers are operated when a fault occurs at a single transmission line between the routers. For example, when cutting of an optical fiber occurs at a transmission line 241b shown in FIG. 43, the routing tables 242b and 242c are updated to change a transmission route for an IP packet.
FIGS. 42D to 42F show examples of routing tables when a transmission line fault occurs. Although the contents of the routing table shown in FIG. 42D are not changed, the IP addresses of the IP routers B and C shown in FIGS. 42E and 42F are changed as indicated by arrows.
With such a structure, if an NT within the LAN C transmits an IP packet 243 addressed to the LAN A, this IP packet 243 is transmitted to the IP router C disposed within the network domain of the LAN C. The IP router C transfers the IP packet 243 to the IP router A, based on the routing table 242c. The IP router A transfers the IP packet 243 to the IP router B, based on the routing table 242a. The IP router B transfers the IP packet 243 to the LAN B, based on the routing table 242b. 
Next, a description will be given in the case where there are two or more transmission lines between routers, with reference to FIG. 44.
FIG. 44 shows how routers are operated when there are three transmission lines between the routers. The transmission lines 244a, 244b, and 244c shown in the figure are capable of transmitting IP packets of 100 Mbps and 1 Gbps by full duplex transmission and are connected between IP routers A, B, and C. Therefore, the maximum bandwidth that the transmission lines 244a, 244b, and 244c can offer is 100 Mbps/1 Gbps×2 (full duplex)×3 (number of transmission lines).
With such a structure, during normal operation, an IP packet 245 passes through routes determined based on the routing tables that the IP routers A to C have, and reaches its final destination. In addition, the fundamental operation during the occurrence of a fault is the same as the case in which the IP routers A to C are interconnected through a single transmission line. When each of the three transmission lines 244a to 244c consists, for example, of three lines, if a fault occurs at one transmission line the bandwidth equivalent to the bandwidth of the damaged transmission line will be reduced. Only when any of the three transmission lines 244a, 244b, 244c between the routers A to C is cut, the routing tables are updated.
When a fault (a line fault, and a transmitter failure within a network) occurs at a network, the routing tables are updated in the IP network 204 and therefore the time to update the routing tables is required. On the other hand, the SONET/SDH network 102 is capable of switching a transmission line to a redundant transmission line at high speeds when a fault occurs.
The SRP is applied to LAN systems and supports only ring-configured networks and is not suitable for star-configured networks that are employed mainly in Ethernet. Because of this, the SRP is devoid of a wide use.
The VRRP supports only units that operate together with LANs, and cannot be applied to other transmission systems. In addition, it takes a few seconds for a working router to be switched from the occurrence of a fault. This is considerable compared with about 50 ms required for switching in the SONET/SDH system.
On the other hand, changing the existing network structures to add the function of healing a fault when it occurs is not practical due to interruption of service, an increase in cost, etc. Because of this, there have been demanded methods capable of healing a fault independently of the configuration of a network or without changing the network configuration.
In addition, because the routing protocol by IP routers changes a route by a protocol level between routers, the time required for line recovery is considerable (typically on the order of a minute or second). Moreover, since there is a need to overwrite information (routing table) about the routes of the whole network, there is still a problem that it will take time for a fault to be completely recovered.
Furthermore, in the case where the number of lines to transmit IP packets is increased, the transmission bandwidth is reduced when a fault occurs. Because of this, when it is judged that a router cannot transmit an IP packet, it is discarded at that router. Thus, when traffic is heavy, there is a problem that transmission service cannot be offered to users with high reliability.